Propelling opposed piston engines into the spotlight

Jon Lawson

Opposed piston (OP) engines have been on the fringes for a long time. Could a new design propel this interesting engine architecture into the limelight? Jon Lawson investigates.

Keying ‘opposed piston engine’ into the worldwide patent office web site generates over 800 results, prompting the question: why hasn’t the technology been more widely adopted? Clearly there is interest out there.

Maybe this is about to change. Two companies have teamed up to create a two-stroke, six-piston, three-cylinder upright gasoline OP engine. What’s more, it is compression ignition.

It‘s a much upgraded second generation version of an existing Achates unit. Laurence Fromm is Executive Vice President, Business Development at the company. He says, “We learned of historic OP engine designs, which have always been more efficient than their conventional engine counterparts. We started the company to modernise the design, using modern tools, supercomputers and powerful design and simulation tools, as well as components such as high pressure common rail fuel injectors, engine control units and precision manufacturing. While always more efficient, no one thought it possible to make a two-stroke engine meet modern emissions standards. In a two-stroke cycle, gas exchange and combustion is one integrated process – modifications to any part of the engine design affects everything. When trial and error was the main development process, it was too time consuming and expensive to design for low emissions and retain high efficiency. What changed – and what sparked the creation of our company – was the development of supercomputers and sophisticated simulation tools, including chemically reactive computation fluid dynamics. The engine features an open and closed cycle operation so it could be simulated on computers and designs could be evaluated and developed analytically, and only then verified and refined experimentally. Even when runs on supercomputers take days to complete, it is still a lot faster and less expensive than designing parts, assembly and testing.”

Fromm sees huge environmental benefits. When asked if OP engines lend themselves to alternative fuels particularly well, he’s very enthusiastic: “Many alternative fuels, like ethanol, methanol and hydrogen can be operated in a low or zero carbon manner, but these high-reactivity fuels do not work well with compression ignition because of their high auto ignition temperatures. Consequently, combustion of low reactivity fuels in conventional engines typically requires spark ignition, which is less efficient than compression ignition. The OP engine has a distinct advantage in operating with low reactivity fuels, including hydrogen – and we can operate with efficient compression ignition across the full operating range of the engine. The reason for this is that our OP engine design has much greater control of trapped conditions, including the portion of residual exhaust gas remaining in the cylinder after the combustion event, and therefore trapped temperatures. This means the OP engine will have both greater power density and greater efficiency than conventional engines when operating with low reactivity fuels. When operating on diesel fuel, it has demonstrated the ability to reduced tailpipe NOx by 90% and CO2 by more than 10% with no additional emissions control technology, and therefore no increase in cost, complexity, or compliance risk compared with today’s engines. So, the engine architecture has important advantages compared with conventional fuels. And because alternative, low-carbon fuels are more expensive than petroleum-based fuels, this improved efficiency can help drive adoption, so we view this as a forever engine.”


To elaborate on what happened next, we talk to Tobias Burek, Chief Engineer at Ricardo.

“While modern four-stroke engines can now surpass OP power density, the fact remains that OP can be very fuel efficient due to the reduced heat transfer. Having two pistons in each cylinder reduces the combustion chamber surface area, resulting in more energy converted to useful torque with a wide efficiency map – the engine operates in a high-efficiency zone for more of its range. Also, OP is very fuel agnostic and high residual heat in the cylinder facilitates compression ignition, which allows us to burn gasoline without a spark.”

The interest in gasoline is due to the US Government’s Advanced Research Projects Agency-Energy (ARPA-E), the body which provided the funding. Burek continues, “A lot of these full-size pickup trucks in the US run on gasoline, so the motivation is to see how efficiently you can burn this as a fuel.”

For delivery, a modified high-pressure common rail diesel system is used, with twin injectors in the middle of the cylinders. On the other side of combustion, there is a Formula One-inspired electrically assisted turbocharger, complete with 48V regen.

So, it’s an opposed piston, two-stroke, gasoline compression ignition hybrid engine. “There’s four levels of novelty!” notes Burek. “It’s a unique engine, especially considering some of the other tech that was developed and integrated. Efficiency, weight, emissions and power have been the themes of this project. Taking efficiency as an example we’ve done some new things such as using 3D printed Inconel pistons (a desirable material due to the high pressures) which enabled us to redesign the cooling gallery of the piston to improve oil flow by 29 per cent over the previous iteration of the engine. We also have fully variable piston cooling jet control. This in turn means we can fit a smaller variable oil pump, improving overall efficiency. Also there’s no auxiliary drive belts on the front, so items such as the water pump are driven by a 48V electric system. We also fitted low friction bearings throughout.”

Around 59 per cent of the weight of the engine has been reduced. This was achieved by large improvements such as redesigning the cranks, counterweights and gears, as well as smaller improvements like further use of plastic for items such as oil pans. Burek explains, “The biggest single weight reduction concerns the cylinder block, so instead of an iron block with wet liners we used a mass production automotive-style aluminium block with spray bore liners. This reduced the weight of that component by 66 per cent.”

It turned out to be one of the most complex castings he has ever been involved with. “With no head gasket to control coolant flow, this is done by cast passages in the engine itself. Getting the right balance took thousands of hours of CFD.” Huge amounts of modelling was done on the project, from air path CFD to optimising scavenging and reducing restrictions to the structural FEA including thermal, fatigue and bore distortion, making sure the piston
rings would seal well.


Nick Fortino is Ricardo’s Project Leader responsible for the test phase. He says, “Despite knock-on effects of the supply chain issues caused by Covid, we managed to get the testing done. Getting all these different complicated technologies like the eturbo, EGR pump, and the various actuators to work together was a challenge. To test, we took 11 steady-state test points covering high frequency portions of the FTP cycle to validate emissions and fuel consumption, with the targets set by a 1D simulation. The fuel consumption side was relatively straightforward, but emissions required more thought.”

This was due to relatively cool exhaust gases. “The EGR doesn’t help here,” continues Fortino. “We solved the problem by placing two oxidation catalyst bricks in the manifold upstream of the turbo, as close to the engine as possible to minimise time to light-off. Downstream of the turbine, there is an electrical exhaust heater, an additional oxidation catalyst, and a SCR system.”

With the desired performance improvements met, the engine has been returned to Achates for further examination. Perhaps these improvements will be enough to see OP engines finally gain a greater market share.

*A heavy duty diesel truck configured with a 10.6L, 3-cylinder diesel opposed piston engine taking part in a demonstration program funded by the California Air Resources Board. Participation in the program showed that the OP engine can reduce tailpipe NOx by 90 per cent (from current US regulations) while reducing fuel consumption (and CO2 emissions) by more than 10 per cent compared with conventional diesel engines.